Two-way wireless communication systems incorporating group dispatch services are known in the art. Group dispatch services typically provide communications within a talkgroup. A talkgroup is a set of logically united subscriber units (e.g., in-vehicle mobile and/or hand-held portable radios) capable of engaging in group-wide communications. In normal talkgroup communications, a single subscriber unit of the talkgroup transmits voice information that is received by a fixed infrastructure and re-transmitted to the other subscriber units in the talkgroup. Typically, such systems use frequency-division multiple access (FDMA) and/or time-division multiple access (TDMA) methods to receive and broadcast the transmission. In FDMA systems, an inbound channel is normally paired with an outbound channel, and the inbound channel can be used by only one subscriber unit at a time. Similarly, in TDMA systems, inbound and outbound time slots are paired and only one subscriber unit at a time can transmit an inbound message for re-transmission.
While these systems provide useful group communications, they force communications to be somewhat regimented in that only one subscriber "owns" the call at any time. That is, more natural group-style communications in which there are simultaneous multiple speakers and multiple listeners are not possible because only one subscriber unit can be transmitting at any time. Although numerous prioritization methods exist today to allow high priority users, or users originating high priority traffic (i.e., emergency calls), to be declared the owner of a talkgroup call, communications are still restricted to single speaker/multiple listener configurations.
Current telephone services provide for conference calls in which multiple speakers and multiple listeners may simultaneously participate in the call. Conference calling can be extended to current wireless systems by allocating a separate inbound channel or time slot to each potential speaker and a separate outbound channel or time slot for potential listeners. Further still, separate full duplex channels (inbound and outbound resources) could be allocated to each subscriber unit of the talkgroup. While these approaches are functional, they are highly inefficient in terms of resource usage. For example, it is not uncommon in public safety organizations to have talkgroups encompassing up to 100 subscriber units. Obviously, establishing a group conference call using separate inbound and/or outbound resources for up to 100 separate subscriber units would require communication resources beyond the capacity of most communication systems.
One solution to the above-mentioned problem is provided by Grube et al. in U.S. Pat. No. 6,005,848, entitled METHOD AND APPARATUS FOR A TALKGROUP CALL IN A WIRELESS CDMA SYSTEM, assigned to the assignee of the present invention and incorporated by reference herein. As described by Grube et al., a sub-talkgroup of the talkgroup is assigned at least one inbound code, with the entire talkgroup assigned an outbound code. Members of the sub-talkgroup can simultaneously transmit voice information using the inbound code(s). The voice information of the talkers is summed and broadcast to all users of the talkgroup. While the above-mentioned technique does provide for a more natural conversation between users, a drawback to the approach is that those individuals that are actively talking, have their voice broadcast to them, which can be annoying to the talker.
Thus, a need exists for duplex communications between members of a talkgroup in wireless communication systems that allows multiple users to simultaneously broadcast to the talkgroup, yet does not allow an active talker to hear their voice broadcast.